Advanced cooling system using throttled internal cooling passage flow for a window assembly, and methods of fabrication and use thereof

ABSTRACT

A window assembly heat transfer system is disclosed in which a window member has a selected transparency to monitored or sensed light wavelengths. One or more passages are provided in the window member for flowing a single-phase or two-phase heat transfer fluid, the passages being optically non-transparent to the monitored or sensed light wavelengths. A mechanism allows either evaporation or condensation of the fluid and/or balancing of a flow of the fluid within the passages. In one embodiment, the window assembly can be made by producing passages in a top surface of a first single plate, optionally producing passages in a bottom surface of a second single plate and bonding the top surface of the first plate to a bottom surface of a second single plate to form the window member with the passage or passages. In another embodiment, the window assembly can be made by providing a core around which the window member material is grown and thereafter removing the core to produce the passage or passages.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/745,927, filed Jan. 17, 2020, now U.S. Pat. No. 10,914,529, and acontinuation of U.S. patent application Ser. No. 16/745,966, filed Jan.17, 2020, and a continuation of U.S. patent application Ser. No.16/745,982, filed Jan. 17, 2020, all of which are incorporated byreference herein in its entirety, and which these applications arecontinuations of U.S. Ser. No. 15/478,474, filed Apr. 4, 2017, now U.S.Pat. No. 10,591,221, all of which is incorporated by reference herein inits entirety. This application is related to co-pending U.S. patentapplication Ser. Nos. 16/747,038 and 16/747,042, the subject matter ofeach is incorporated herein by reference. This application is related toco-pending application entitled ADVANCED COOLING SYSTEM USING THROTTLEDINTERNAL COOLING PASSAGE FLOW FOR A WINDOW ASSEMBLY, AND METHODS OFFABRICATION AND USE THEREOF having the first named inventor Tucker,Brian P. having the Ser. No. 17/187,998 and being filedcontemporaneously.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates generally to thermally controlled systemsfor infrared and other wavelength optical windows that are exposed toextreme high and low temperatures and their production, such systemsallowing their use in these extreme environments by actively cooling orheating the window, respectively, via flow passages internal to thewindow. In the case of window heating, that situation would typically berequired to prevent icing and/or condensation since, in general, a coldwindow does not hinder transmission.

Infrared (IR) windows are currently unable to survive the environmentexperienced by hypersonic vehicles traveling above Mach 3. Windowtemperatures can exceed 900° C. from exposure to the high heat fluxesand temperatures associated with hypersonic flight. A typical prior artIR window 102 on a section of a hypersonic vehicle designated generallyby numeral 100 employing a sensor 103 (schematically illustrated), suchas an IR sensor, is shown in FIG. 1. The high-speed air 104 flows alongor against the exterior surface 105 of the window, thereby generatingheat which will destroy the window or render the window opaque to thewavelength of light being sensed or monitored by the sensor 103.

Optical windows and domes used in hypersonic aircraft, missile systemsand spacecraft for infrared imaging or other wavelength imaging demandgood mechanical stability and high optical transmission in thewavelength range of the sensor that is viewing through the window. Forinfrared sensors, the wavelength range is typically between 0.4 micronand 12 microns. Zinc sulfide, zinc selenide, germanium, galliumarsenide, gallium phosphide, and cadmium telluride are used inapplications such as IR windows which require long wavelength infrared(LWIR) optical transmission capability. The fabrication of zinc sulfideand zinc selenide via chemical vapor deposition (CVD) routes is onepossible pathway. Alternatively, IR windows can be produced by forming agreen body from a population of nanoparticles, depositing a layer of ZnSpowder and sintering the covered green body to produce a sinteredproduct.

One known window assembly consists of an outer window and inner windowwith an intervening space which can be filled by a materialcharacterized by high thermal insulation properties or, alternatively, acooling fluid is circulated through an intervening space so that, theentire intervening space between the outer and inner windows is filledwith either an insulating gas or a cooling fluid. There are significantdrawbacks to this approach. Since the sensor must look through thecooling or insulating fluid in the intervening space, the inner windowis coated with an optical coating that is substantially transparent atthe visible and/or the infrared frequency portion of the electromagnetspectrum to reduce reflections because the two liquid-to-windowinterfaces can cause reflection or variation of the wavelength,resulting in noise in the IR signal and reduced quality. Yet anotherdrawback of this type of assembly is that, because the interior flowcross-section created by the intervening space is simply a wide-openunsupported area, it has less of an ability to withstand a largepressure gradient between the fluid in the intervening space and theinterior or exterior of the window. Also, the open flow cross-section ofthe intervening space means that there are no defined flow passages orflow paths in intervening space, but rather only an open cross-section.Therefore, without any such flow constraints, uniform flow across theintervening space is problematic in that fluid flow can be expected totake the shortest path from the inlet to outlet, leaving other areaswith little or no flow, resulting in large temperature variations acrossthe window. While this type of prior art proposed the coolant as asingle-phase liquid or gas, flow maldistribution would have been an evengreater problem if an evaporating fluid had been contemplated, becausethe highly variable g-forces experienced in a hypersonic vehicle ormissile will further exacerbate this flow maldistribution and potentialflow instabilities. This is because the inertial effects on the liquidand vapor in the coolant passage will be different.

Another known window assembly includes an inner window, an outer window,and a support subsystem between the inner and outer windows defining aplurality of infrared transparent fluid flow cooling channels forcooling the outer window without adversely affecting the opticalproperties of either window. Like the assembly just discussed, thissecond known approach also proposes to have transparent fluid flowcooling passages. We have recognized, however, that it isdisadvantageous for wavelengths of interest for the sensor to betransmitted through the cooling passages as that will adversely affectthe sensor data and also that opaque cooling passages are a solution tothat problem.

A conceptual silicon window cooled using single-phase water has alsobeen described by Wojciechowski et al (Internally Cooled Window forEndoatmospheric Homing, AIAA, May 1992). We have found, however, thatthe use of single-phase water causes undesirably lower heat transfercoefficients and requires equally undesirable large mass flow rates tomaintain a uniform temperature. Furthermore, the silicon cooled windowconcept is only applicable to MWIR since silicon substrate absorbs inLWIR.

Zinc sulfide is a common material for LWIR and semi-active laser windowsand domes. Multi-spectral zinc sulfide (ZnS), made by CVD, is alsocommercially available. A sintering process is believed to produce amore erosion-resistant and ultra-high density IR window. Regardless ofthe base material being used, or the manufacturing process used to formthese windows, they must be actively cooled because of the very highthermal loads that result from the high-speed airflow over the exteriorduring flight. Currently available IR window materials are still unableto withstand the heat fluxes and temperatures associated with theseconditions. Future missile performance improvements will be impededwithout the development of advanced cooling strategies for high-speedweapon windows.

We have discovered a way to address these problems by employing aninternally cooled IR window that uses two-phase flow in one or morechannels having hydraulic diameters less than about 0.118 inch (3 mm)within the window to maintain both a uniform temperature, a low massflow rate (relative to single-phase flow which results in lower pressuredrops) and a low temperature while also minimizing system size. Thepassages are optically non-transparent to a monitored or sensed lightwavelength. An inlet flow restriction or valve such as a orifice, weir(partial dam of the cross-sectional flow area), capillary tube section,reduced hydraulic diameter section or adjustable throttling valve (suchas a thermostatic expansion valve (TXV) or electronic expansion valve(EXV)) is provided at the inlet of each flow passage in the IR window toact as a throttling device upstream of the evaporation section of thecooling passages which will, for two-phase evaporating coolants,initiate adiabatic nucleation (i.e., evaporation) and prevent boiling(evaporation) hysteresis. This flow restriction will also act as a flowbalancing mechanism for two-phase coolant flow and even single-phasecoolant flow, since the pressure drop across this restriction willsubstantially exceed pressure drops due to the flow down an individualpassage, pressure variations due to orientation, and pressure variationsdue to external g-forces. For two-phase coolants, the pressure drop ofthe flow restriction will make perturbations in the pressure drop in thetwo-phase coolant passageway, i.e., perturbations potentially resultingfrom variations in the thermodynamic quality due to variations in heatloading applied to the window, insignificant relative to the pressuredrop across this upstream throttling device. This throttling device canbe located at the inlet of the IR window, at the interface between thewindow and the inlet flow manifold or at the exit of the inlet manifold.Such a configuration keeps the flows in balance at the design flowrates, regardless of changes in g-forces, orientation, and thermodynamicflow quality due to non-uniform heat loading on the coolant passages orthe window itself. In addition, either an upstream (upstream of the IRwindow) control or throttling valve can be used along with the passageflow restrictions to control the superheat or quality exiting theactively cooled window. As part this discovery process, we haveconsidered the effects of material light absorption and emission,channel emission, channel diffraction, and temperature-induced wavefrontdeformation. At the same time, we have discovered as well, a novelmethod to fabricate these windows with internal cooling passages forusing two-phase flow. One of many advantages of our discoveries is thatthey provide significantly better performance than any previousinternally cooled window that has been demonstrated in the prior artsuch as those which use two-phase ammonia impinging onto a porous coverand vent coolant directly in front of the window (which cansignificantly degrade image quality) or require bonding of the window, astainless-steel coolant tube, and a porous cover.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of our invention willbecome more readily apparent from the following detailed descriptionwhen taken in conjunction with the accompanying figures wherein:

FIG. 1 depicts a known type of IR window used as part of a section of ahypersonic vehicle, such as a missile, rocket or high speed airplane,employing a sensor which views through the IR window as above described.

FIGS. 2a, 2b and 2c are, respectively, embodiments of an actively cooledIR window used on a hypersonic vehicle, such as a missile, rocket orhigh speed airplane, employing a sensor which views through the activelycooled window which has parallel-flow passages, series-flow passages ora combination of parallel- and series-flow passages.

FIG. 3 is a schematic drawing of an open-loop IR window cooling systemconfiguration where liquid coolant stored in the reservoir flows throughthe passages in the IR cooled window to prevent any thermally relateddamage to the IR window.

FIG. 4 is a schematic drawing of a closed-loop IR window cooling systemconfiguration where liquid coolant in the actively cooled window is theevaporator of a vapor compression cooling loop.

FIG. 5 is a schematic drawing of a closed-loop pumped two-phase IRwindow cooling system configuration where liquid coolant in the activelycooled window is the evaporator of a pumped two phase thermal controlloop.

FIG. 6 is an isolated schematic view of an embodiment of our inventionin which an internally cooled IR window is fabricated frommulti-spectral zinc sulfide (ms-ZnS) or single-crystal Zinc Sulfideplate-like material.

FIGS. 7(a)-(d) show a currently contemplated method according to ourinvention to obtain high aspect ratio inner channels by CVD growth,after polishing, continuing after placement of a selectively removablecore-channel forming material.

FIGS. 8(a)-(e) shows another embodiment of a method according to ourinvention to obtain the same result as in FIG. 7(a)-(d) but with removalof the polishing step between the two CVD growth steps to form the innerchannel material around the core material.

DETAILED DESCRIPTION OF THE DRAWINGS

An embodiment of a novel window designated generally by numeral 200 inaccordance with our invention is shown in FIG. 2a . Once again,hypersonic or simply high speed airflow 204 over or against the outersurface causes heat generation on the vehicle surface and the window. Werecognized that to keep the window from failing or preventing propervisibility to a known type of sensor 203 (schematically shown) requiresthat the window be actively cooled. In this currently contemplatedembodiment of the present invention, one or more cooling passages 201″-2mn″ (mn being used herein to designate the last of any number of desiredparts in a series) are arranged inside the window 200 to provide coolingto maintain the window at a survivable temperature, so that nearisobaric evaporation of the coolant inside the passages 201″-2 mn″ alsoprovides near isothermal temperature distribution given that theevaporation temperature is directly related to the evaporation pressurewhich is nearly isobaric. This nearly uniform temperature distributiontranslates into less thermal distortion of the wavelength being observedby the sensor 203. A small restrictor or adjustable throttling valve canbe arranged at the entrance to each passage (shown in FIGS. 3, 4 and 5)or, alternatively, in an interface between a manifold and the entranceof the passages to act as a throttling device upstream of theevaporation section of the cooling passages to initiate nucleation andalso as a flow balancing system to prevent flow instabilities, such asmanometer tube parallel-path flow oscillations to name but one possibleflow instability being prevented. The sensor 203 can, for example, be along wavelength Infrared (LWIR) sensor, medium wavelength infrared(MWIR) sensor, a short wavelength infrared (SWIR) sensor or even avisual sensor or camera. It will now be understood in the art that,while only parallel-flow passages have been shown as an exemplaryembodiment, series-flow passages (211″ in FIG. 2b ) or combinations ofseries- and parallel-flow passages (221″, 222″ in FIG. 2c ) can also beused within the scope of the present invention.

In one currently preferred embodiment of our invention, the activelycooled window is an IR window cooling system which is cooled by theevaporation of the coolant flowing through the IR window and can beconfigured in either a closed-loop or open-loop system. Of course, itwill now be appreciated by one skilled in this art that all types ofwindows can employ the present invention, and that a window transparentto LWIR is perhaps the most challenging application of our technologydue to limited available material options and the processing abilitiesof those available materials.

With regard to the type of window assembly shown in FIG. 2a , anopen-loop IR window cooling system configuration designated generally bynumeral 300 is shown in FIG. 3 where coolant stored in a reservoir 321flows through an optional control or throttling valve 323 then enters aninlet flow distributing manifold 324 before being directed into theindividual flow passages 301″ through 3 mn″ located inside the IR window302. An inlet flow restriction or valve 301′ through 3 mn′ such as aorifice, weir, capillary tube section, reduced hydraulic diametersection or adjustable throttling valve (such as a TXV or EXV) isprovided at the inlet of each flow passage in the IR window to act as athrottling device upstream of the evaporation section of the coolingpassages 301″ through 3 mn″ which will initiate adiabatic nucleation(i.e., evaporation) and prevent boiling (evaporation) hysteresis, andalso to act as a flow balancing system since the pressure drop acrossthis restriction will outweigh the pressure drops due to the two-phaseflow down an individual passage, thereby making perturbations in thepressure drop in the two-phase coolant passageway (potentially as aresult of variations in the thermodynamic quality) insignificantrelative to the pressure drop across this upstream throttling device.This throttling device can be located at the inlet of the IR window, atthe interface between the window and the inlet flow manifold or at theexit of the inlet manifold. Even if a single-phase coolant fluid is usedas the cooling fluid in the embodiments disclosed herein andcontemplated as part of our inventive concept, such a configurationkeeps the flows in balance or at the design flow rates, regardless ofvariations in g-forces, orientation, and thermodynamic flow quality(caused by non-uniform heat loading on the coolant passages or thewindow itself). Either the optional control or throttling valve 323 orthe individual flow restrictions 301′ through 3 mn′ or both can be usedto control the superheat or quality exiting the open loop system at theoutlet to the environment 330. In a currently preferred embodiments ofour invention, a variety of two-phase coolants or mixtures thereofincluding water, ammonia, liquid nitrogen, liquid helium, liquid neon,liquid argon, liquid krypton, liquid xenon and other liquefied gases aswell as conventional refrigerants including hydrofluorocarbon (HFC)refrigerants, perfluorocarbon refrigerants, hydrocarbon (HC)refrigerants and hydrofluro-olefin (HFO) refrigerants can be used inthis configuration as one skilled in the art will now recognize.

The coolant evaporates either partially or completely and then exits theindividual cooling passages 301″ through 3 mn″ being combined in an exitflow manifold 325 before flowing into an optional evaporation pressureregulator 322 and then vented either overboard 330 or internally to thevehicle carrying the window assembly. It is well understood by oneskilled in the art that the evaporation pressure regulator could bereplaced by a temperature or flow regulating valve or that a combinationof pressure, temperature and flow can be used to determine the positionof the valve 322. It is also understood that the overall cooling couldbe initiated by opening either valve 323 or 322 and terminated by thecomplete closing of either valve 323 or 322. Also, the person of skillin the art will understand the greater thermal inertia and thermalcapacity that is provided to the IR window if valve 322 is used thecontrol turn on and turn off when compared to using valve 323 to performthose functions. Likewise, one versed in the art will understand thatlower pressure is developed in the IR window cooling passages if valve323 is used the control turn on and turn off as compared to using valve322 to perform those functions.

FIG. 4 displays another embodiment of our closed-loop IR window coolingsystem configuration designated generally by numeral 400 where liquidcoolant flowing from a condenser 463 flows through an optional controlor throttling valve 423 then enters an inlet flow distributing manifold424 before being directed into individual flow passages 401″ through 4mn″ located inside an IR window 402. At the inlet of each flow passagein the IR window there is an inlet flow restriction or valve 401′through 4 mn′ such as a orifice, weir (partial dam of thecross-sectional flow area), capillary tube section, reduced hydraulicdiameter section or adjustable throttling valve (such as a TXV or EXV)to act as a throttling device upstream of the evaporation section of thecooling passages 401″ through 4 mn″ which will initiate adiabaticnucleation (i.e., evaporation) and prevent boiling (evaporation)hysteresis and also act as a flow balancing system since the pressuredrop across this restriction will outweigh the pressure drops due to thetwo-phase flow down an individual passage, thereby making perturbationsin the pressure drop in the two-phase coolant passageway (potentially asa result of variations in the thermodynamic quality) insignificantrelative to the pressure drop across this upstream throttling device.This throttling device can be located at the inlet of the IR window, atthe interface between the window and the inlet flow manifold or at theexit of the inlet manifold. Again, even if a single-phase coolant isused, such a configuration keeps the flows in balance or at the designflow rates, regardless of changes in the thermodynamic flow quality dueto non-uniform heat loading on the coolant passages or the windowitself. Either the control or throttling valve 423 or the individualflow restrictions 401′ through 4 mn′ or both can be used to control thesuperheat or quality exiting an exhaust manifold 425. The evaporatedcoolant leaving the IR cooled window 402, hereafter also referred to asthe refrigerant, will then flow to a suction line accumulator 461 toassure that only superheat refrigerant vapor enters a compressor 462. Ifthe refrigerant exiting the IR window 402 via the exhaust manifold 425is typically always saturated and not superheated, then a pumpedtwo-phase loop configuration such as shown in FIG. 5 can be used insteadof the vapor compression system shown in FIG. 4, or heat can be added tothe suction line accumulator 461 (or heat added to an additional heatexchanger or recuperator upstream of the compressor 462) prior to thecompression of the refrigerant vapor to avoid the steady-state injectionof incompressible liquid refrigerant by the compressor 462.Alternatively, if the refrigerant flow exiting the exhaust manifold 425is typically superheated or is assured to be superheated by feedbackcontrol of this information by either the optional control or throttlingvalve 423 and/or the individual flow restrictions 401′ through 4 mn′ orboth then the superheat of refrigerant normally exiting the exhaustmanifold 425 or exiting the passages before entering the exhaustmanifold will evaporate any transient saturated liquid that may enterthe suction line accumulator, due to transient operation. In this way,any short-term transient liquid, due to the rapid changing of powerlevels or valve settings can be accommodated by the temporary hold-up ofthis liquid refrigerant in the suction line accumulator 461. A varietyof two-phase coolants or refrigerants including water, ammonia, liquidnitrogen and other liquefied gasses as well as numerous potentialconventional HFC, HFO and HC refrigerants can be used in thisconfiguration as one skilled in the art will now recognize. However, ourcurrently preferred embodiment of two-phase cooling employs two-phaseevaporation of water.

The condenser 463 shown in FIG. 4 is air cooled, and the air flowthrough the condenser can be increased with an optional fan or blowerassembly 464. One skilled in the art will understand, however, that thisair-cooled condenser can be replaced by a liquid cooled condenser withinthe scope of our invention. It is also understood that a typical vaporcompression system has other common and well-known components such as afilter-drier, high and low pressure safety controls, and liquid receiverto name just a few that would be present in any typical vaporcompression system.

FIG. 5 displays yet another embodiment of our invention in the form of aclosed-loop pumped two-phase IR window cooling system configurationdesignated generally by numeral 500 where liquid coolant flowing from apump 591 through an optional control or throttling valve 523 then entersan inlet flow distributing manifold 524 before being directed into theindividual flow passages 501″ through 5 mn″ located inside an IR window502. At the inlet of each flow passage in the IR window there is aninlet flow restriction or valve 501′ through 5 mn′ such as a orifice,weir, capillary tube section, reduced hydraulic diameter section oradjustable throttling valve (such as a TXV or EXV) to act as athrottling device upstream of the evaporation section of the coolingpassages 501″ through 5 mn″ which will initiate adiabatic nucleation(i.e., evaporation) and prevent boiling (evaporation) hysteresis andalso act as a flow balancing system since the pressure drop across thisrestriction will outweigh the pressure drops due to the two-phase flowdown an individual passage, thereby making perturbations in the pressuredrop in the two-phase coolant passageway (potentially as a result ofvariations in the thermodynamic quality) insignificant relative to thepressure drop across this upstream throttling device. This throttlingdevice can be located at the inlet of the IR window, at the interfacebetween the window and the inlet flow manifold or at the exit of theinlet manifold. Such a configuration keeps the flows in balance or atthe design flow rates, regardless of changes in the thermodynamic flowquality due to non-uniform heat loading on the coolant passages or thewindow itself. Either the control or throttling valve 523 or theindividual flow restrictions 501′ through 5 mn′ or both can be used tocontrol the superheat or quality exiting an exhaust manifold 525. Thetwo-phase or superheated refrigerant (coolant), leaving the IR cooledwindow 502 will then flow to a condenser 563. Only liquid refrigerantshould exit the condenser 563 so that it can be pumped, withoutcavitation, by the liquid pump 591. The condenser 563 shown is aircooled in the illustrated embodiment and the air flow through thecondenser 563 can be increased with an optional fan or blower assembly564. One skilled in the art will again understand that an air-cooledcondenser can be replaced by a liquid cooled condenser within the scopeof our invention. It is also understood that a typical two-phase pumpedloop system has other well-known common components such as a pressurizerand liquid receiver to prevent pump cavitation and to control thecondensation and resulting evaporation temperatures to name just a fewthat would be present in any modern two-phase pumped loop system. Oneskilled in the art will now further recognize that a single-phase pumpedcooling loop could also be employed within the scope of our inventioninstead of our currently preferred pumped two-phase cooling loop using,by way of example only, water. The pumping power in the single-phasepumped cooling system would, however, be greater due to the requirementfor additional mass flow rate of the single-phase coolant. A variety oftwo-phase coolants or refrigerants including water, ammonia, liquidnitrogen and other liquefied gasses as well as numerous potentialconventional HFC, HFO and HC refrigerants can be used in thisconfiguration as one skilled in the art will also now recognize.

We have investigated materials that can be fabricated with internalcooling passages and used for our active cooling window as shown inTable 1 below. Unfortunately, most materials that transmit LWIR are alsoincapable of handling the high temperatures and stresses associated withhypersonic flight so active cooling is necessary. It is well known inthe art that candidate materials that do not absorb LWIR at all arediamond, germanium, zinc selenide, and gallium arsenide. Germanium isunsuitable for a hypersonic environment because free-carrier absorptionrenders the window useless above 100° C. and a high thermo-opticcoefficient (400×10⁻⁶/° C.) significantly distorts the signal duringhigh heat fluxes. Zinc selenide (ZnSe) is generally consideredunsuitable for supersonic applications because its low hardness makes itvery susceptible to erosion. However, its low thermo-optic distortionmakes it an excellent material for applications that do not requiredurability. Gallium arsenide (GaAs) is a good candidate if the windowtemperature remains below the free-carrier absorption limit of 400° C.,and also has the capability to be used in the MWIR wavelength region.However, GaAs is susceptible to wavefront deformation because of a highindex of refraction change with temperature (dn/dT=150×10⁻⁶/° C.) andlow thermal conductivity (48 W/m-K). Of course, our two-phase activethermal control is able to reduce the temperature gradient of thewindow, thereby mitigating optical distortion caused by the high indexof refraction change with temperature. As was known prior to ourinvention, diamond is an ideal material for the hypersonic environmentin that it has the best thermal conductivity, lowest thermo-opticcoefficient, and highest hardness but its use is currently limited bycost, availability, and a maximum temperature of 700° C. (as set by theoxidation limit).

Since some absorption of the IR signal can be tolerated, five additionalmaterials can be considered. Float zone silicon (FZ-Si) offers higherosion resistance and strength and has previously been used. Galliumphosphide (GaP) provides good erosion resistance and minimal wavefrontdeformation but has high LWIR absorption and is not commerciallyavailable. ZnS offers improved erosion resistance relative to ZnSe atthe cost of increased temperature-induced wavefront distortion and adecrease in performance from absorption. Tuftran™ (ZnSe with a ZnScoating), combines the low absorption of ZnSe with the improved erosionresistance of ZnS. However, the low strength and thermal conductivity ofthe ZnSe layer leads to problems with thermal shock at high heat fluxes.The mismatch in the thermal expansion coefficients of the two materialsalso can lead to delamination of the layers. Finally, our currentlypreferred embodiment for a LWIR window uses multi-spectral Zinc Sulfide(ms-ZnS) which is sold under the registered trademark “CLEARTRAN” © ofDow Chemical and is formed by modifying ZnS to be water free by a hotisostatic pressing process because it removes most LWIR absorption andall MWIR absorption at, however, the expense of some reduced strengthand hardness. Gallium arsenide (GaAs) is another good candidate for MWIRwindows as long as the window temperature remains below the free-carrierabsorption limit of 400° C., which of course is possible with the activethermal control disclosed in this invention.

TABLE 1 Properties of LWIR Window Materials Float Zone Gallium Property[Units] Germanium ZnSe GaAs Diamond Silicone Phosphide ZnS ms-ZnSThermal Conductivity 59 18 48 2000  159 110 17 27 [W/m-K] dn/dT 408 61149   15.6 150 137 41 54 Strength [MPa] 93 55 138 ≈300   125 ≈100   10369 Maximum Temperature 100 400 700 260 600 600 600 [° C.] Absorption inMWIR at No No No Very No No High No Room Temperature High Absorption inLWIR at No No No No High High Moderate Low Room Temperature CommerciallyYes Yes Recently High Yes No Yes Yes Available Cost Knoop Hardness 692105 750 8260  1150 840 210 150 [kg/mm²]

Many of the materials listed in Table 1 above also perform well in theMWIR band. The ms-ZnS is also a good MWIR material because it does notabsorb at all in this region. If improved performance is desired, FZ-Sialso does not absorb in the MWIR region while offering higher mechanicalstrength and a higher thermal conductivity (less coolant channelblockage because channels can be spaced further apart). AdditionalMWIR-only materials are listed in Table 2 below. Sapphire is desirablebecause of its thermal conductivity, hardness, and strength. However, italso absorbs significantly at high temperatures. At 427° C., a 2 mmthick sapphire window has an emittance of 0.34, which can significantlyaffect performance. Yttria has minimal high-temperature absorption buthas issues with thermal shock because of low thermal conductivity andrelatively low strength; it is also not yet commercially available. Wecurrently contemplate using one or more of the above materials informing the window according to the processes described hereinbelow.

TABLE 2 Properties of MWIR Window Materials Property (Units) YttriaSapphire ALON Spinel MgF₂ Thermal Conductivity 13.5 36 12.6 14.6 14.7[W/m-K] dn/dT 30 6-12 2.8 3 1 Strength [MPa] 160 300 300 190 125 Maximum— — — — — Temperature [° C.] Absorption in MWIR Low Moderate ModerateVery Low at Room Temperature High Absorption in LWIR Very 100% 100% 100%Very at Room Temperature High High Commercially Yes Yes Recently HighInter- Available Cost national Knoop Hardness 720 2200 1800 1600 580[kg/mm²]

Temperature adversely affects a window in three ways. The first is thatmost IR window materials absorb some of the signal passing through thewindow, and this absorption increases with the temperature of thewindow. For example, a 0.8 cm thick ms-ZnS window at room temperatureabsorbs 9% of the LWIR energy passing through it. However, at 150° C.,it will absorb 15% of the energy passing through and at 600° C., it isexpected that 29% of the energy passing through is absorbed. The secondis that, like any surface at elevated temperature, the window radiatesfar more energy, and at the higher temperatures the energy radiatedactually limits performance more than wavelength absorption. Using thedefinition of normalized Target Acquisition Range (TAR) as the actualTAR (i.e., the distance that a target becomes visible to an IR sensorthrough a window) divided by the unencumbered or ideal TAR (i.e., thedistance at which a target is visible if no window were present), at awindow temperature of 600° C., the actual TAR is reduced to only 48% ofthe ideal TAR (normalized TAR is 0.48). The third is that many materialswill oxidize or decompose at high temperatures, rendering the windowopaque or even destroying the window entirely. It is well known in theart that, for IR sensor windows to be used in hypersonic applications,active cooling is absolutely necessary. Up until our present invention,however, the problem of useful active cooling had not been solved.

Our invention is specifically engineered so that the wavelength of thesensed or monitored light that happens to strike the cooling passagesrather than the window area between the passages is not transmittedthrough the passages. For improved image quality and to avoiddistortion, we have been able to avoid passing the wavelengths of lightof interest to the sensor or being monitored through the coolingpassages and to assure that they pass only through the area where thereare no cooling passages. The cooling passages are configured to benon-transparent to the wavelengths of light being sensed or monitoredby, for example, applying a coating to the inside surfaces of thepassages, using passages with opaque walls (such as hollow cores asdescribed below) and/or other techniques for preventing transmission.Even if the liquid coolant fluid is already opaque, in a two-phasecoolant application, the vapor may not be transparent, or the differentindex of refractions between the liquid and vapor coolant can lead tosignal distortion, noise in the sensed signal, and other optical issues.Our invention allows the use of any evaporating two-phase fluid,transparent or non-transparent, because of the absorbing coating orother type of coating that is applied to the inside of the passage wallsto make the surface non-transparent and/or absorb any radiation (light)in the sensor wavelength, and thereby provide a clear sensor signalwhile avoiding any optical signal noise caused by transmission of thewavelengths through the passage and to the sensor. This coating on theinside surfaces, as well as exploiting any inside passage surface finishbenefits that can be obtained by our core manufacturing process, avoidany issues associated different indexes of refraction at the passage'sfluid-solid interface or evaporating liquid-vapor interface.

Our invention also allows the use of two-phase evaporation of theworking fluid which results in far lower mass flow rates, improvedtemperature uniformity along the passage length, and significantlyhigher heat transfer coefficients, which thereby permit the use ofsmaller (optically opaque to the sensed wavelengths) passages, makingless of the window frontal area non-transparent to the signal beingmonitored or sensed. By way of example, one currently preferredembodiment of our invention uses two-phase water coolant in a ms-ZnSLWIR window, where the cooling passage geometry has been optimized forLWIR operation. The ZnS window material is modified to be water-free bya known hot isostatic pressing process thereby forming ms-ZnS. Thisembodiment has similar calculated performance relative to an uncooledLWIR window up until the uncooled window begins to oxidize at 650° C.and is rendered useless, demonstrating that active cooling is essential.Our cooling passages in the LWIR window have been shown to reduce theactual TAR of the sensor located behind the window to half the ideal TARwhen operating at room temperature.

We have also found that, by using two-phase evaporative water cooling tocontrol the window temperature instead of single-phase sensible watercooling, our system achieves both improved isothermality, and requiresthat the volume of coolant needs to be only 1% of the volume that wouldbe required for a single-phase water system that exhibits the samethermal performance. Our invention is able to use far smaller passageswithout experiencing an excessive and unwanted pressure drop, as well asa far smaller liquid water reservoir in the case of an open system, andfar smaller overall system in the case of a closed vapor compressionsystem or two-phase pumped loop system. We have also found that even theuse of two-phase ammonia evaporation as an alternative two-phase coolantin our invention will increase the actual TAR by 20% (i.e., by a factorof 1.2) although the volume of cooling fluid required is increased by afactor of 3.

FIG. 6 shows an embodiment of our internally cooled IR window that hasbeen fabricated from plate-like material, transparent to the wavelengthsbeing sensed or monitored. A window 600 with internal flow passages isformed by bonding a base plate 681 with a cover plate 682. The baseplate 681 has uniform internal coolant passages 601″ through 6 mn″machined into the surface of the base plate 681. While the cover plate682 can also be machined with fluid passage grooves, a flat cover plate602 is bonded to the base plate 681 in the illustrated embodiment toform the passages 601″ through 6 mn″. By way of one specific example, wemanufactured 4.7 inch (long)×4.1 inch (wide) LWIR windows that can beused in the FIG. 3, 4 or 5 embodiments from both ms-ZnS andsingle-crystal zinc sulfide material. Each window contained thirty-threecoolant passages of 0.020 inch (0.5 mm) width×0.060 inch (1.5 mm) depth[i.e., with a hydraulic diameter of 0.030 inch (0.75 mm)], and a passagelength of 4.7 inches (119.4 mm). The inlet of each of these flowpassages contained identical inlet capillary flow restrictions with areduced hydraulic diameter section that spanned the initial 0.109 inch(2.77 mm) length of the inlet passage. The diameter of the reducedhydraulic section creating the upstream flow restrictions in the IRwindow cooling passages varied from 0.007 inch (0.178 mm) to 0.0105 inch(0.267 mm). While rectangular windows have been shown in the figures andin this example, one skilled in the art will now recognize, that thewindow geometry could be square, round, oval or essentially any shape.Likewise, one skilled in the art will now recognize, that the coolingpassages could be round, square, rectangular, triangular or essentiallyany shape.

In addition to the use of adhesives to bond window plates together toform the coolant passages, if the two surfaces are highly polished andif the particular window configuration would allow edge clamping ormechanical fixturing to hold the two polished surfaces mechanicallytogether, these polished surfaces would not leak and can be used to forma actively cooled window. Furthermore, the window plates could be bondedvia optical contacting or diffusion bonding processes which are wellknown in the art.

We have discovered that, after the cooling passages are fabricated, thepreferred method is to etch a rough surface, oxidize the surface, and/ordeposit an optical absorbent material on the inside surfaces of thecooling passages so that any IR radiation that hits the cooling passagesis absorbed or scattered rather than being transmitted through thecoolant passages which might cause IR sensor discrimination issues.

Our preferred method of producing the window is to grow the windowmaterial around a solid or hollow core, to form the desired passages. Anexample of a growth method is chemical vapor deposition (CVD) of groupII/VI compounds, such as zinc sulfide. If the core is hollow andnon-transparent to the signal being sensed or monitored, the core can beused as the channel. If the core is solid, the core can be removed,still allowing conversion or coating of the surface and leaving anon-transparent channel surface. The core can be chosen such that it canbe removed selectively by an etching process, thus leaving one or morepassages for coolant flow. Acids attack most group II/VI CVD grownmaterials, therefore materials which can be dissolved by a base solutionare preferred for the core material. The core to be selectively etchedcan be a metal such as aluminum so as to be selectively removed with anaqueous copper chloride solution. Inasmuch as an amphoteric material canbe dissolved by an acid or base, one such as aluminum oxide or coppercan be chosen for the core material and dissolved with a base such asaqueous sodium hydroxide. Further, if the core is not adherent to the IRmaterial and has a coefficient of thermal expansion (and contraction)greater than that of the IR material then the core material may beremoved by cooling (the window and core) and then pressing the core outof the window with simple mechanical force (if the core extends to theedges of the window). This process is not limited solely to theproduction of IR windows.

Alternatively, since IR windows can be produced by forming a body from apopulation of nanoparticles, depositing a covering layer of ZnS powderon the body and sintering the covered body to produce a sinteredproduct, the core can be imbedded into the body, that is formed aroundthe core. In this way, after the sintering of the green body, the corecan then be mechanically or chemically removed, in the same or similarfashion used in the CVD fabricated window with internal coolantpassages.

For window designs where the individual passages extend to the edges ofthe material (after core removal), the coolant passages interface withboth inlet and outlet manifolds external to the window. Alternatively,the core can be configured to contain both the inlet and outletmanifolds, in addition to the individual passages, and the entire coretotally encapsulated by the grown IR window material, so that the inletand outlet passageways can be later drilled from the window exteriorinto the inlet and outlet manifolds (from either the top, bottom, oredges) to allow the etching solution to remove the core through theseinlets and outlets that were created by drilling. We have also foundthat the window can be machined and/or the core removed by chemicaldissolution either before or after the machining and/or hydrostaticpressing operations.

One currently contemplated method to form the material around aselectively removable core is shown in FIGS. 7(a)-(d) and designatedgenerally by the numeral 7000. In this embodiment, a group II/VImaterial 7100 is grown by CVD to a desired thickness. If a very thicklayer is grown and “alligator skin” effect of large surface moieties7101 form (a), then they must be polished to obtain a flat surface.Stopping CVD growth, followed by polishing and then continuing CVDgrowth was previously demonstrated by Purohit et. al. when growing ZnSon CVD-grown ZnSe. [: Purohit, Kirsch and MacDonald; Method forpreparing laminates of ZnSe and ZnS. 1990]. In addition, gas or solutionphase chemical treatment may be employed to functionalize the surfacefor continued, or a second, CVD growth step of the material. Prior tothe second CVD growth step a core material 7202 is placed (supported orunsupported by a frame) atop the polished CVD grown substrate 7201. ThenCVD growth is continued around the core to form a core-containing CVDblank 7301(c). The cores (only one being designated by numeral 7302)then can be removed by chemical etching or mechanical processing,leaving behind fluid flow passageways or channels (only one in (d) beingdesignated by numeral 7402) in the CVD blank 7401 that may requireadditional polishing of the “alligator skin” moieties 7303 of the typealso shown in FIG. 7c . Once the core is removed, the interior surfacesof the channels or passages can be treated to make them non-transparentto the sensed or monitored wavelengths, such as treating them for IRabsorbance.

Another contemplated novel method for implementing our inventiondesignated generally by numeral 8000 is to form the material around aselectively removable core and to potentially remove the need forpolishing the moieties 7101 in FIG. 7. As shown in FIGS. 8 (a)-(e) theCVD starter blank is grown from a single crystal, atomically smooth,wafer 8201 with the core material 8202 already affixed to the wafersurface (a). After sufficient growth occurs, the core-containing CVDcrystal 8304 (b) can be cleaved from the wafer 8301 to leave behind acore/host piece 8304 with a near-atomically smooth surface 8305 so thatthe piece can then be flipped (c) and CVD growth on the smooth side 8305can continue without the need for mechanical polishing or treatment (d).After completion of growth, the core material 8302 can be removed aspreviously discussed and any moieties 8303 on the outer surface can bepolished, leaving behind a CVD grown structure 8401 with open channels8402 (e).

Yet another currently contemplated method is to begin with a slab of theZnS (either multispectral or single crystal) or some other suitablewindow material as previously listed and discussed, and then optionallypolished one or both surfaces. One or more fluid passage forming coresare then placed on the slab, and material is formed around theselectively removable core. In this embodiment, a group II/VI materialis grown by CVD to a desired thickness around the core forming acomplete window. As stated before, if a very thick layer is grown and“alligator skin” effect of large surface moieties form, then thesemoieties can be polished to obtain a flat surface. Once again, gas orsolution phase chemical treatment may be employed to functionalize theslab surface for the CVD growth step of the material. While ourcurrently preferred embodiment is to grow the same group II/VI materialas the base slab material, the base material and the grown materialcould be different a group II/VI materials, and other methods to buildup the material over the cores in addition to CVD can be used.

Another window material growth method could employ sintering hostparticles together to a poly- or single-crystalline structure asperformed by Ravichandran et. al. [2: Ravichandran and Shi;Polycrystalline sintered nano-gran zinc sulfide ceramics for opticalwindows. (U.S. Pat. No. 8,803,088 B1)], by only hot isostaticallypressing the particles around the core material and following similarremoval procedures such as mechanical force or selective chemicaletching.

Other variations in the method of adding inlet and outlet manifolds canbe employed either external or internal to the window. In the case ofmaterials such as ms-ZnS, the hot isostatic pressing can be performedafter the window, with its internal passages, has been created andbefore or after the core is removed by etching or other knowntechniques. The surface finish of the core can be adjusted to thedesired surface finish inside the passages after the core is removed.The inside surfaces of the passages can utilize any non-transparent orabsorbing coating, and one way to deposit this coating on the interiorsurfaces of the passages is by flowing a carrier liquid (with theoptical coating in solution) through the passages, then evaporating thecarrier liquid, with or without the addition of heat, to leave theoptical coating bonded to the interior surfaces of the coolant passages.Inlet flow restrictions in the window can also be formed by the core bycreating a core where the inlet section of the core has a reduceddiameter so that, after core removal, a smaller inlet flow passagewayhas been created.

We have discovered that chemical vapor deposition (CVD) is one effectiveway of growing the ms-ZnS window with internal passages. The growth rateis 50-75 μm/h, making the fabrication method economically viable as wellas technically feasible. One skilled in the art will now understandthat, while the actively cooled window discussed here is an IR windowthat allows an IR sensor located behind the window to be protected fromthe very high environmental temperatures and aerodynamically inducedthermal loads while flying at high speeds, this same window coolingtechnology has other applications for both window heating or windowcooling where a transparent window is exposed to high or low temperatureenvironments or excessive heating or cooling loads. Of course, oneskilled in the art will further understand that for heating loads,condensation rather than evaporation would be used to heat rather thancool the window. For example, the present invention is useable forwindows for human-viewing or sensor-viewing from structures located inextreme environments, such as extraterrestrial planetary missions,spacecraft reentry sensors, specialty high temperature furnace sensorapplications, high temperature geological investigations and the like.

While currently preferred embodiments of the invention have beenillustrated and described, variations will be apparent to one skilled inthe art without departing from the principles of the invention describedherein. Therefore, we do not intend to be limited to the details shownand described above but intend to cover all such changes andmodifications as are encompassed by the scope of the appended claims.

We claim:
 1. A window system for a sensor, comprising: a monolithicmaterial having a transmissivity to selected wavelengths of light to bemonitored or sensed by the sensor; at least one passage arranged withinthe monolithic material; and a fluid in the at least one passage thatrenders the at least one passage optically non-transparent to themonitored or sensed light wavelengths.
 2. The window system of claim 1,wherein the at least one passage is comprised of a plurality of passagesconfigured as one of parallel-flow passages, series-flow passages, and acombination of parallel- and series-flow passages.
 3. The window systemof claim 1, wherein the material is selected to be transparent to thelight wavelengths selected from at least one of long wavelength IRlight, mid-wavelength IR light, short wavelength IR light, and visiblelight.
 4. The window system of claim 1, wherein the material is one ofmulti-spectral zinc sulfide, single crystal zinc sulfide, zinc selenide,germanium, gallium arsenide, cadmium telluride, diamond, float zonesilicone, or gallium phosphide.
 5. A window system for a sensor,comprising: a monolithic material having a transmissivity to selectedwavelengths of light to be monitored or sensed by the sensor; at leastone passage arranged within the monolithic material; and an opticallynon-transparent to the monitored or sensed light wavelengths fluid inthe at least one passage.
 6. The window system of claim 5, wherein theat least one passage is comprised of a plurality of passages configuredas one of parallel-flow passages, series-flow passages, and acombination of parallel- and series-flow passages.
 7. The window systemof claim 5, wherein the material is selected to be transparent to thelight wavelengths selected from at least one of long wavelength IRlight, mid-wavelength IR light, short wavelength IR light, and visiblelight.
 8. The window system of claim 5, wherein the material is one ofmulti-spectral zinc sulfide, single crystal zinc sulfide, zinc selenide,germanium, gallium arsenide, cadmium telluride, diamond, float zonesilicone, or gallium phosphide.
 9. A window system for a sensor,comprising: a monolithic material having a transmissivity to selectedwavelengths of light to be monitored or sensed by the sensor; at leastone passage arranged within the monolithic material; and a fluid in theat least one passage, wherein the fluid hinders the selected wavelengthsof light to be monitored or sensed by the sensor passing through the atleast one passage.
 10. The window system of claim 9, wherein the fluidin the at least one passage that is hindering the selected wavelengthsof light to be monitored or sensed by the sensor passing through the atleast one passage is opaque to the selected wavelengths of light to bemonitored or sensed by the sensor.
 11. The window system of claim 9,wherein the at least one passage is comprised of a plurality of passagesconfigured as one of parallel-flow passages, series-flow passages, and acombination of parallel- and series-flow passages.
 12. The window systemof claim 9, wherein the material is selected to be transparent to thelight wavelengths selected from at least one of long wavelength IRlight, mid-wavelength IR light, short wavelength IR light, and visiblelight.
 13. The window system of claim 9, wherein the material is one ofmulti-spectral zinc sulfide, single crystal zinc sulfide, zinc selenide,germanium, gallium arsenide, cadmium telluride, diamond, float zonesilicone, or gallium phosphide.